Thank you to all the responders. 

When I checked again, after initiating a return, the "send message", button was active.

I've now used both avenues to contact the seller, who has accepted a "return" but hasn't responded to my request to send the missing box of pins. It leaves me wondering if they misunderstood my request -- "english language" ?

While it's tempting to select on price alone, I usually compare similar items to give me an indication of quality control and shop setup. Then i filter by country and point of manufacture if possible. It's usually not hard to find the sellers that put in the effort and factor in their costs. So, yes I do like to support local sellers and I appreciate that freight and other overheads affect their pricing. I'd rather pay that bit extra and I hope that it helps genuine sellers to stay in the marketplace. 

Ebay is failing to provide the tools that continue to improve trade, to their detriment.